In the field of medical diagnosis and industrial inspection, inspections using a radiation inspection apparatus such as an X-ray computed tomography apparatus (X-ray CT apparatus) are in practice. The X-ray CT apparatus has a structure that an X-ray tube (X-ray source) emitting a fan-shaped X-ray beam and an X-ray detector in which plural X-ray detecting elements are arranged are disposed to face each other with a sectional plane of a subject as a center between them. In the X-ray CT apparatus, the fan-shaped X-ray beam is emitted from the X-ray tube to the X-ray detector, and an angle is varied by one degree at a time with respect to the sectional plane every time the beam is emitted to obtain X-ray absorption data. The X-ray absorption data is analyzed by a computer to calculate an X-ray absorption rate of each position on the sectional plane, and an image is constructed according to the X-ray absorption rates.
The X-ray detector in the X-ray CT apparatus includes a solid state scintillator which emits visible light when excited by X-rays. The solid state scintillator is a monocrystalline scintillator or a polycrystalline ceramic scintillator. An X-ray detector composed of the solid state scintillator and a photodiode is now under development. By using the X-ray detector using the solid state scintillator, it is easy to increase the number of channels while making the detecting elements compact, and the X-ray CT apparatus can be made to have higher resolution.
Known examples of the solid state scintillator used for the X-ray detector include monocrystalline bodies such as cadmium tungstate (CdWO4), sodium iodide (NaI) and cesium iodide (CsI), and polycrystalline ceramics such as europium-activated barium fluorochloride (BaFCl:Eu), terbium-activated lanthanum oxybromide (LaOBr:Tb), thallium-activated cesium iodide (CsI:Tl), calcium tungstate (CaWO4), cadmium tungstate (CdWO4), europium-activated gadolinium oxide (Gd2O3:Eu) and praseodymium-activated gadolinium oxysulfide (Gd2O2S:Pr).
Rare earth oxysulfide ceramics such as (Gd1-xPrx)2O2S(0.0001≦x≦0.01) and (Gd1-x-yPrxCey)2O2S(0.0001≦0.01, 0≦y≦0.005) have characteristics such as a large X-ray absorption coefficient and a short afterglow time of light emission. Apart from the above, rare earth oxide ceramics having a garnet structure is also known as a solid state scintillator. The rare earth oxide ceramics having a garnet structure has a characteristic that light output is excellent. However, the X-ray CT apparatus is desired that an exposure dose of a subject is further reduced. Therefore, it is demanded that the solid state scintillator material is more sensitive and its afterglow time is reduced in order to reduce the scan time.
On the other hand, as a measure for terrorism prevention in the airport attracting attention in these days, the solid state scintillator most commonly used in the security fields such as a luggage inspection system or the like is a monocrystalline body of cadmium tungstate. In comparison with the rare earth oxysulfide ceramics or the rare earth oxide ceramics having a garnet structure, the monocrystalline body of cadmium tungstate is inferior in characteristics but superior in view of cost because its cost is low. But, the monocrystalline scintillator of cadmium tungstate has a possibility of worsening the environment because cadmium (Cd) is a noxious substance.